RALEIGH — Duke Energy shareholders are calling on the company’s board to launch an independent investigation into issues surrounding a massive coal ash spill that coated 70 miles of a North Carolina river in toxic sludge.

A letter sent to Duke’s board of directors Thursday by a coalition of more than 20 large institutional investors says their confidence has been shaken by the Feb. 2 spill into the Dan River. The letter also expresses concern about an ongoing federal criminal investigation and what the investors characterize as the company’s inadequate response to the environmental disaster.

The group is comprised of directors of big pension plans that hold Duke stock, including state treasurers in Connecticut, Oregon and Pennsylvania, as well as the Illinois State Board of Investment and California State Teachers’ Retirement System.